Rice husk valorisation by in situ grown MoS2 nanoflowers: a dual-action catalyst for pollutant dye remediation and microbial decontamination

Rice husk (RH) is a common agricultural waste generated during the rice milling process; however, a major portion is either burned or disposed of in landfills, posing significant environmental risks. In this study, RH waste was transformed into bio-based catalysts via delignification cum in situ growth of MoS2 (DRH-MoS2) for efficient pollutant dye removal and microbial decontamination. The developed DRH-MoS2 exhibits nanoflower-like structures with a 2H-MoS2 phase and a narrow band gap of 1.37 eV, which showed strong evidence of photocatalytic activity. With the presence of abundant hydroxyl functionality, delignified rice husk (DRH) exhibits a malachite green (MG) dye adsorption capacity of 88 mg g−1. However, in situ growth of MoS2 nanosheets on DRH enhances MG degradation to 181 mg g−1 under dark conditions and 550 mg g−1 in the presence of light. Mechanistic insights reveal a synergistic adsorption-cum-degradation phenomenon, amplified by generation of reactive oxygen species during photodegradation which was confirmed from radical scavenging activity. Interestingly, DRH-MoS2 demonstrates potent antibacterial activity against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) with sustained photodegradation efficiency (>80%) over three cycles. The present work reports a cost-effective and scalable strategy for environmental remediation of real wastewater which usually contains both dye pollutants as well as microbes using abundantly available renewable resources such as sunlight and agricultural biomass wastes.

using MultiPak 9.9 version (ULVAC-PHI), and deconvolution of the peaks was carried out using Origin software (2023b).

X-ray diffraction (XRD)
The synthesised composite DRH-MoS 2 and DRH powder was subjected to XRD analysis using MiniFlex 600 (Bench Top XRD instrument, Rigaku, Japan).The powdered samples were then evenly distributed onto aluminium holders with a flat glass spreader.Cu-k α radiation is generated from a copper anticathode used for the analysis and operated at 40 kV voltage and 20 mA current.Scanning was done from 2 (θ) angles ranging from 5° to 90° at a scan rate of 5° per minute and a step size of 0.02.The Segal equation (eq.1),Bragg's equation (eq.2), and Scherrer equation (eq.3) were used to calculate the crystallinity index (CI) and the distance between crystal planes (d-spacing) and crystal size (D), respectively.

Scanning electron microscopy (SEM)
A Carl Zeiss SEM (ZEISS EVO) instrument from Germany was used to examine the morphological properties of the DRH and DRH-MoS 2 composites at an accelerated voltage of 10 kV.The samples were carefully mounted on double carbon tape before being coated with a layer of gold and palladium using a desk sputter coater (DSR1, United Kingdom).For 200 seconds, the coating process was carried out under a vacuum of 200 torrs.Image J software was used to determine the particle size distribution of the MoS 2 component.Further, Energy Dispersive Spectroscopy (EDXS, USA) was used to analyse the elements present on the sample surfaces (at an accelerated voltage of 10 kV) and mapped using Team Basic software for studying the distribution of in-situ grown MoS 2 within the DRH.

Adsorption kinetics
The kinetic model is a mathematical representation of the relationship between the extent of contact and the amount of adsorbate adsorbed onto the adsorbent.Pseudo-first-order kinetic (eq.5) and pseudo-second-order kinetic (eq.6) models were used to investigate the interaction of the solid adsorbent (DRH and DRH-MoS 2 ) and the liquid adsorbate (MG dye), as well as calculate the rate of adsorption.The active sites on the adsorbent have the most significant influence on the adsorption rate, providing insights into the behaviour and mechanism of the adsorption process.Both models' linear equations are provided below. (eq.4) l (  -  ) = log   - 1  (eq.5)

𝑄 𝑒 𝑡
Where and (mg/g) is the amount of MG (mg) adsorbed on DRH and DRH-MoS 2 at     equilibrium and at any time (t), (min -1 ) and (g mg −1 min −1 ) are the rate Pseudo-first-order  1  2 kinetic and pseudo-second-order kinetic models respectively.

Adsorption isotherm
Adsorption primarily concerns transferring dyes from a liquid phase (aqueous solution) to a solid phase (adsorbent).The transfer process can be described mathematically using equilibrium adsorption isotherms, which present the interaction between the dye and the adsorbent surface.As a result, Langmuir and Freundlich isotherms (eq.6 and 7) were investigated, and mathematical representations of both isotherms are provided: (eq.6) where (mg/g) and is the adsorption constant or the maximum adsorptive capacity for  0 Langmuir isotherm and adsorption rate respectively.is Freundlich isotherm constant and   is the sign for describing the favourability of the adsorption process.

Adsorption thermodynamic
Adsorption thermodynamics was used to determine the characteristics of the adsorption process, such as whether it was endothermic or exothermic and whether it followed a spontaneous or non-spontaneous mechanism.The thermodynamic properties of the adsorption process, such as Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), were calculated using equation (9) and equation ( 10), and a plot of lnk c against 1/T was generated.The universal gas constant (R) is 8.314 J/mol.K and the absolute temperature expressed in Kelvins (T) were used in these calculations.

UV-visible diffused reflection spectra (DRS)
The UV-visible diffused reflection spectra (DRS) analysis of DRH-MoS 2 was conducted utilizing the Lambda 750 spectrophotometer, PerkinElmer, USA.The determination of the band gap for DRH-MoS 2 was carried out using Tauc equation (eq.12).
'A' remains a constant in the equation.

Antioxidant activity of the DRH-MoS 2
The DPPH free radical scavenging experiments were performed according to the literature by Hong et al. 1 .Firstly, 0.3 Mm DPPH was prepared in ethanol.Different concentrations of DRH-MoS 2 (0.1-0.25 mg/ml) were prepared in both ethanol and distilled water and sonicated for 1 min at 27°C.0.5 mL of prepared DRH-MoS 2 samples were mixed in 0.5 mL DDPH solution and incubated in the dark for 30 minutes.The percentage of DDPH inhibition of the sample was calculated using equation 11. (eq.11) The absorbance of DPPH in ethanol is represented as A c , while A b signifies the absorbance of the sample in the absence of DPPH, and A s is the absorbance of the sample in the presence of DPPH after a 30-minute dark incubation.

Effect of time on the formation of MoS 2 on DRH
MoS 2 gown at the optimized temperature reported in our previous study using sugarcane biomass at 195°C for 18 hours was selected for this study with slight modification.On utilizing DRH as substrate for in-situ synthesis of MoS 2 , we found that after 17 hours of reaction at 195°C, the white colour of DRH turn to grey and in the SEM image sheet or flower like morphologies was not discovered.However, after18 hours of reaction at 195°C, DRH colour turns to light black but still the nanosheets or nanoflower like morphologies were absent in the SEM micrographs.After 19 hours of reaction at 195°C, DRH turns to complete black and nanoflower like morphologies were observed in SEM micrographs of MoS 2 (Figure S3).From these results we confirmed the growth and formation of MoS 2 nanoflowers on to the microstructures of DRH which occurs at 195°C for 18 hours.
peak I 002 corresponds to the crystalline plane, while the intensity peak I am represents the peak for the amorphous region at an angle of 2θ around 18.6°.The symbol λ represents the wavelength of Cu K α radiation, and n signifies the number of wavelengths.The parameter d refers to the spacing between two adjacent layers of atoms.

Table S1 :
Represents the elemental composition of DRH and DRH-MoS 2 determined through XPS spectroscopic studies.

Table S2 :
Crystal structure properties of RH, DRH and DRH-MoS 2 determined through XRD.

Table S3 :
Represents pseudo first-order and pseudo second order kinetic parameters for the adsorption of MG onto DRH.

Table S4 :
Details of the adsorption isotherm parameters for DRH

Table S5 :
Represents the adsorption thermodynamic parameter for DRH

Table S6 :
List of pseudo first-order and pseudo second-order kinetic parameters of DRH-MoS 2 for photodegradation of MG at different concentrations.

Table S8 :
Comparison of present work and summary of dye degradation capabilities of MoS 2 27 and its composite from literature, encompassing details on dosage quantity, percent degradation 28 of dyes, process parameters and duration.